Fuel cell technology shows great promise as an alternative energy source for numerous applications. Several types of fuel cells have been constructed, including polymer electrolyte membrane fuel cells, direct methanol fuel cells, alkaline fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide fuel cells. For a comparison of several fuel cell technologies, see Los Alamos National Laboratory monograph LA-UR-99-3231 entitled Fuel Cells: Green Power by Sharon Thomas and Marcia Zalbowitz.
FIG. 1 represents an example of a fuel cell 100, including a high surface area anode 110 including an anode catalyst 112 (typically platinum alloy), a high surface area cathode 120 including a cathode catalyst 122 (typically platinum), and an electrolyte 130 between the anode and the cathode. The electrolyte may be a liquid electrolyte; it may be a solid electrolyte, such as a polymer electrolyte membrane (PEM); or it may be a liquid electrolyte contained within a matrix, such as the electrolyte in a phosphoric acid fuel cell (PAFC).
In operation of the fuel cell 100, fuel in the gas and/or liquid phase is brought over the anode 110 where it is oxidized at the anode catalyst 112 to produce protons and electrons in the case of hydrogen fuel, or protons, electrons, and carbon dioxide in the case of an organic fuel. The electrons flow through an external circuit 140 to the cathode 120 where air, oxygen, or an aqueous oxidant (e.g., peroxide) constantly is being fed. Protons produced at the anode 110 travel through electrolyte 130 to cathode 120, where oxygen is reduced in the presence of protons and electrons at cathode catalyst 122 to produce water in the liquid and/or vapor state, depending on the operating temperature and conditions of the fuel cell.
Hydrogen and methanol have emerged as important fuels for fuel cells, particularly in mobile power (low energy) and transportation applications. The electrochemical half reactions for a hydrogen fuel cell are listed below.
Anode:2H2→4 H+ + 4 e−Cathode:O2 + 4 H+ + 4 e−→2 H2OCell Reaction:2 H2 + O2→2 H2OTo avoid storage and transportation of hydrogen gas, the hydrogen can be produced by reformation of conventional hydrocarbon fuels. In contrast, direct liquid fuel cells (DLFCs) utilize liquid fuel directly, and do not require a preliminary reformation step of the fuel. As an example, the electrochemical half reactions for a Direct Methanol Fuel Cell (DMFC) are listed below.
Anode:CH3OH + H2O→CO2 + 6 H+ + 6 e−Cathode:1.5 O2 + 6 H+ + 6 e−→3 H2OCell Reaction:CH3OH + 1.5 O2→CO2 + 2 H2O
These reaction schemes illustrate the production of water at the cathode during operation of these fuel cells. The water content of the cathode during fuel cell operation is affected by several factors, including production of water due to the normal course of the reduction reaction at the cathode, electro-osmotic drag of water from anode to cathode associated with proton (H+) flow, and production of water from the oxidation of fuel that has crossed through the electrolyte to the cathode instead of reacting at the anode. If allowed to accumulate, liquid water from these processes can severely limit the rate at which further gaseous oxidant reaches the catalyst surface, resulting in an undesirable condition referred to as “cathode flooding”. Consequently, water is typically removed from the cathode as vapor in the oxidant gas flow stream and either is vented from the system or is condensed external to the fuel cell. If desired, the recovered water may then be supplied to the anode. In general, the amount of water lost as vapor should be equal to the amount needed to stay in stoichiometric neutrality. If excess water in either the liquid or gaseous phase is vented from the system, then additional water must be provided to the fuel cell to avoid dehydration. If the water vapor is condensed for recycling within the system, the fuel cell can have significant undesirable parasitic losses associated with high operating oxidant stoichiometries. Moreover, the presence of an external condenser in the system can introduce additional weight, parasitic losses, and complexity to the fuel cell.
The performance of conventional DMFCs may suffer more than hydrogen fuel cells due to “methanol crossover,” in addition to cathode flooding from water production and electro-osmotic drag. The material used to separate the liquid fuel feed from the gaseous oxidant feed in a DMFC is typically a stationary PEM that is not fully impermeable to methanol or other dissolved fuels. As a result, methanol fuel may cross over the membrane from the anode to the cathode, reacting with the cathode catalyst directly in the presence of oxygen to produce heat, water and carbon dioxide but no useable electric current. In addition to being an inherent waste of fuel, methanol crossover causes depolarization losses due to a mixed potential at the cathode and, in general, leads to decreased cell performance.
It is desirable to provide a system for controlling the amount of water at or within the cathode in a fuel cell where liquid water can potentially accumulate and inhibit oxygen transport. Preferably such a system would prevent and/or buffer the system against cathode flooding and, if needed, would recover the water produced by the fuel cell without the addition of significant parasitic losses or of increased system complexity in order to maintain water neutrality. It is also desirable to provide a fuel cell in which fuel crossover is minimized.